The present invention relates to embodiments of a parking brake controller and method to maintain an air supply to a trailer, such as a trailer used in an air braked commercial vehicle. In a parking brake system for an air braked vehicle, the driver pushes a dash mounted tractor park switch when he wants to release the parking brake of the tractor. The driver pushes the dash mounted trailer supply switch when he wants to supply air to the trailer. Supplying air to the trailer will release the parking brakes of the trailer. The driver pulls the tractor park switch when he wants to release air from the parking brake actuators to apply the parking brakes of the tractor. The driver pulls the trailer supply switch when he wants to evacuate air from the trailer air supply to apply the parking brakes of the trailer. In a multi-function valve, such as the MV-3® dash control module from Bendix Commercial Vehicle Systems LLC, the supply air to the trailer is automatically evacuated when the air is released from the parking brake actuators of the tractor. Therefore, the parking brakes of the trailer are applied at approximately the same time as the tractor. However, there are situations when the driver may want to maintain the air supply to the trailer to keep the parking brakes of the trailer released even after the tractor parking brakes are applied.